As known in the art, communication systems were originally developed to provide voice communication services. As technology has evolved, communication systems now provide data services and various multimedia services in addition to the voice communication services. However, conventional communication systems, which were developed to provide voice communication services, have a narrow bandwidth for data transmission and require a high subscription fee, thus failing to satisfy diversified user demands. Furthermore, in conjunction with the rapid development in the communication industry and ever-increasing demands for Internet services, it is important to provide communication systems capable of providing Internet services efficiently. As a result of these trends, broadband wireless communication systems have been proposed to provide efficient Internet services.
Broadband wireless communication systems use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme. Thus, broadband wireless communication systems can provide high-rate data transmission by transmitting physical channel signals using a number of subcarriers. The standardization of wireless access schemes for broadband wireless communication systems is being conducted by the Institute of Electrical and Electronics Engineers (IEEE), which is an international standardization organization, particularly by the IEEE 802.16 standardization group.
Ranging is a kind of random access process in a broadband wireless communication system. The ranging process is used to control time/frequency offsets between a base station (BS) and a user terminal and to correct the transmission (TX) power of the user terminal. The ranging process can be divided into initial ranging, periodic ranging, bandwidth request ranging, and handover ranging, depending on the purpose.
A ranging code (or code set) used in each ranging process is broadcast using an Uplink Channel Descriptor (UCD) message. A user terminal attempts ranging by transmitting an intended ranging code through a ranging region detected from an uplink MAP (UL-MAP) message. A base station uses the received ranging code to calculate time/frequency offsets, and transmits a RaNGing ReSPonse (RNG-RSP) message containing an offset correction value to the user terminal. That is, through a code ranging process, the user terminal corrects physical time/frequency offsets with the base station. The ranging code transmitted by the user terminal in the ranging process is called a Code Division Multiple Access (CDMA) code. The CDMA code is generated by a pseudo random binary sequence (PRBS) generator.
If a base station supports a plurality of frequency assignments (FAs), loads may be considerably different between FAs. If loads are considerably different between FAs, the system needs to balance a cell load, and a user terminal needs to move to a small-load FA in order to secure a good service quality. Cell load estimation is essential for such cell load balancing. However, in the conventional art, cell load estimation is performed unilaterally by the base station, and a user terminal only operates under the control of the base station.
If a user terminal can estimate a cell load actively, the user terminal can control communication states more actively on the basis of the estimated cell load.
However, the conventional user terminal cannot control communication states more actively because it cannot estimate a cell load by itself.